Zinc alloy coatings, having a high level of aluminum, impart increased corrosion protection to ferrous metals and improve formability as well as paintability, compared to conventional hot-dip zinc alloys. However, a high concentration of aluminum makes the coating process very sensitive to metal surface conditions. Consequently, the successful application of aluminum-rich zinc alloy coatings has been limited to some relatively expensive and sophisticated processes, such as the double dip process with standard galvanizing preceding an aluminum-zinc coating, using a suitable alloy such as GALFAN™, a trademark of International Lead Zinc Research Organization, Inc., which contains nominally 5% Al; the electro-fluxing process whereby electroplating with a thin zinc layer precedes the GALFAN™ alloy coating; and the hot process where a furnace with a reducing atmosphere is used before applying the GALFAN™ alloy coating.
Coating problems also persist at much higher aluminum concentrations (such as eutectoid compositions containing 22% Al) and with various other zinc alloy bath compositions (such as those containing vanadium, manganese, magnesium, silicon, tin, bismuth and nickel). Such specialty alloys are found to be incompatible with conventional zinc chloride/ammonium chloride fluxes. Certain patents, such as U.S. Pat. No. 6,200,636 and U.S. Pat. No. 6,284,122, attempt to deal with coating issues by deposition of a metallic layer by cementation on the steel surface. However, the successful application of such a method depends on cementing a layer of copper requiring a pristinely clean steel surface which is very difficult to achieve in practice.
Numerous attempts to apply GALFAN™ alloy coating in a traditional single-stage process have failed. Conventional fluxes used for aluminum-rich zinc alloy hot-dip galvanizing have resulted in uncoated spots, pinholes, surface roughness and bad adhesion. Special fluxes have been developed to overcome these problems. For example, U.S. Pat. No. 1,914,269 describes a galvanizing flux composition which contains ammonium chloride, zinc chloride and fluorine compounds. U.S. Pat. No. 3,806,356 discloses a pre-flux containing various combinations of fluorosilicic acid, hydrochloric acid, hydrofluoric acid, potassium fluoride and zinc chloride. U.S. Pat. No. 4,496,612 proposes an aqueous flux based on zinc chloride, ammonium chloride and from 0.6 to 3.0% fluoride ions. All of these fluxes contain acutely toxic fluorides which are hazardous to workers and the environment.
U.S. Pat. No. 4,802,932 discloses a fluoride-free top flux for kettles containing 80 to 90% ZnCl2; 0 to 20% NH4Cl; and, based on the weight of ZnCl2+NH4Cl, 0.01 to 5% of a wetting agent, 0 to 5% of a foaming agent, and 0 to 5% of a soluble salt of rare earths.
EP 0 488 423 B1 suggests an aqueous flux composition which comprises 10 to 50% by weight of zinc chloride and/or stannous chloride; 1 to 20% by weight of at least one alkali metal chloride or alkaline earth metal chloride; and 0.1 to 30% by weight of at least one alkyl quaternary ammonium salt wherein the alkyl groups have 1 to 18 carbons.
The flux formulations of the above-cited U.S. Pat. No. 4,802,932 and EP 0 488 423 B1 were tested on galvanizing with GALFAN™ coatings but none of them gave good results with the coatings having high roughness, pinholes and sometimes uncoated (bare) spots.
The present invention is based on analysis of the chemical processes at the surface of steel samples after pickling in acid and fluxing. It is common practice to rinse articles with water after pickling in preparation for galvanizing. While advancing to the fluxing tank, moist surfaces, which have become very active by pickling, are exposed to air. Even though the articles are carefully washed, some iron salts still remain on the surface. Therefore, the surfaces after pickling may have active iron atoms and molecules of FeCl2 which are rapidly converted by air into FeOHCl and Fe(OH)2. Applicant has found that at least one of these compounds, Fe(OH)2, cannot be dissolved in flux solution at pH>1.5. Consequently the surfaces contain Fe(OH)2 which reacts with aluminum in the molten zinc-aluminum alloy, creating aluminum oxides, which are not wetted by the molten alloy, according to the following reaction:3Fe(OH)2+4Al→3Fe+2Al2O3+3H2  (1)
This peculiar feature of galvanizing in the presence of aluminum results in unsatisfactory coatings with bare spots and pinholes.